100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 353

Diploids are potentially useful in guayule breeding because of their sexual
(non-apomictic) reproduction and thus the ability to use standard breeding meth-
odologies. While there are problems in using diploids, such as significantly lower
yields and increased susceptibility to root diseases, these yield and disease prob-
lems have been overcome by using modified recurrent selection schemes to
increase yield and mass selection to develop Verticillium- tolerant lines [ 19 , 23 ].
These improved diploid lines can either be crossed to apomictic polyploids or have
their chromosome numbers doubled with colchicine. Diploids could also be used to
release new genetic combinations by crossing them as the female parent to apo-
mictic polyploids. The resulting apomictic progeny plants might contain new and
useful combinations of genes, because meiosis in the microspore mother cells of the
apomictic polyploid male plants is normal. Once high-yielding polyploids are
identified, they could be crossed onto diploids resulting in populations with enough
variation from which to make selections [ 1 , 5 , 19 ].
A potential breeding method that can make the most of limited resources in
guayule is the pedigreed natural crossing method [ 27 , 28 ]. Guayule meets the
requirements for use of this method by having natural cross pollination between
potential parents (species or diploids or polyploids) and dominant markers to
identify hybrids. The advantages of this method are that crossing is not dependent
on limited time available for a single scientist or trained assistant to perform the
cross; identification, harvesting, and isolation of hybrids can be done by semiskilled
workers on land unsuitable for yield trials and other experiments; and it is more
economical than making crosses in the greenhouse. The biggest disadvantages are
that the pedigree of the hybrids is based on a parental line rather than a single plant
and large amounts of land may be needed to identify hybrids. The advantages of this
method of producing large numbers of hybrids with little effort should outweigh the
disadvantage of individual parent plant identification. The higher outcrossing rate
of guayule compared with self-pollinated species should result in a larger number of
hybrids being identified with the same amount of land.
Traits of Interest
The primary objective for all guayule breeding programs to date has been to
increase rubber yield. Secondary objectives have included improving rubber qual-
ity, resin yields, seedling and mature plant vigor, plant architecture, regeneration
following harvest by clipping, and tolerance to salinity, drought, diseases, and pests
[ 1 , 5 , 15 , 19 ]. However, because of the relatively few researchers involved in
guayule breeding, the secondary objectives have not received much attention
over time.
Selection in guayule has been significantly aided by the description of the
components of yield and their relationships to rubber production [ 29 , 30 ]. In
general, rubber content (%) was not positively correlated with rubber yield and in
fact was often negatively correlated. Fresh and dry weights, as well as other